A Tropical Depression over the South China Sea in June 2025—Observational and Forecasting Aspects

A tropical depression (TD) formed over the northern part of the South China Sea and affected Hong Kong during 25–26 June 2025. Based on the historical database, there were not many TDs following a similar track in the past, namely, a northwestward track towards Hainan Island and the Leizhou Peninsula. This paper serves to document a number of aspects of the forecasting service for this TD, including: (1) consideration of the upgrade of the system from a tropical disturbance to a TD, possible further upgrade into a tropical storm, and the location of the centre in “multiple centre” situation (broad tropical cyclone centre and a mesocyclone embedded in the convection near the centre); (2) forecasting of the intensity and the impact on local winds; and (3) wind structure analysis based on dropsonde and wind profiler data. Moreover, this case demonstrates that artificial intelligence models are proven to provide earlier alerting of the possible occurrence of this TD and its subsequent movement towards the coast of southern China, whereas the conventional physics-based models remain useful in the forecasting of the impact of TD on the winds in Hong Kong for the operation of the tropical cyclone warning signal services.